New OFDM System without Guard Interval OFDM新方法

9.3-2 New OFDM System without Guard Interval

Yafei Hou, Member, IEEE; Tomohiro Hase, Member, IEEE

Research Center for Information and Communication Systems, Ryukoku Univ. Japan. yafeihou@ieee.org and hase@rins.ryukoku.ac.jp .

Abstract-- This paper proposes a new OFDM system without a guard interval (GI) based on the useful characteristics of IFFT operation. The system utilizes the pilot signal to capture the channel information as well as GI does. Therefore, the bandwidth efficiency of the system can be improved. In addition, it achieves promising peak-to-average power ratio (PAPR) characteristics which also decrease the cost of handsets.

I. INTRODUCTION Orthogonal frequency division multiplexing (OFDM) is a promising solution for high data rate transmission in frequency-selective fading channels. It achieves high bandwidth efficiency for applications including wireless multimedia, wireless internet access, future generation mobile communication systems, consumer electronics and power-line communication systems. The OFDM system often utilizes a cyclic prefix (CP) as a guard interval (GI) which must be at least as long as the channel impulse response (CIR) to avoid inter-symbol interference (ISI) and inter-channel interference (ICI). This is a severe restriction in systems with a short latency time and high bandwidth efficiency requirements. Several attempts have been made to overcome the problem such as shortening the CIR or nor using a guard interval by using the redundancy in the frequency domain. This paper proposes a new simple OFDM system without a guard interval based on the useful characteristics of IFFT operation. The proposed system benefits the design of handsets. In addition, it achieves promising peak-to-average power ratio (PAPR) characteristics which also decrease their cost.

II. A USEFUL CHARACTERISTIC OF IFFT OPERATION Suppose x[n] is a message-bearing subcarrier symbol. We assume that another symbol sample exists and K is an integer. The sample is constructed from by the following formula

(1) Here can be any fixed value chosen from For each , -point IFFT operation can be represented as

(2)

where . We assume that, and . Formula (2) can also be represented as

(3) Here is constant for each fixed value

; is independent of parameter , and is an -point IFFT operation on symbol . For each fixed value ( = 0,), let us use to represent a vector which holds the -point samples where can be any fixed value chosen from. We can find other kinds of vector, or all the time-domain samples

can be reconstructed using any one -point vector with and

fixed vector. Since all the time-domain samples, can be reconstructed using any -point vector,

, the transmitter can only utilize an -point vector of each to modulate the symbol

. Thus it can reconstruct new time-domain samples after times for

modulated symbols. Therefore the maximum PAPR value of -point time-domain OFDM samples only depends on an -point IFFT operation on symbol which will reduce the PAPR. On the other hand, from in Formula (2), all kinds of the -point vector will be identical if equals zero, that is

. Since the equi-space pilots inserted in the frequency domain will get a better channel estimation [1], this property can be utilized to choose ) as the time-domain of the pilot signal mixed into all modulated symbol to capture the channel information.

After obtaining the channel information, such pilot signals can be removed after the equalizer at the receiver and IFFT operations. In addition, the can be used to design a constant training sequence to realize the correct timing of the OFDM system [2]. In this paper, we utilize these characteristics to design an OFDM system without guard interval.

978-1-4244-2559-4/09/$25.00 ©2009 IEEE III. MODEL OF OFDM SYSTEM WITHOUT GUARD INTERVAL Figure 1 shows the proposed structure of time-domain OFDM symbol and its model over a multipath channel. Suppose each OFDM symbol can transmit -QAM data symbols and samples for the pilot. We assume that the maximum channel delay occupies samples duration and is smaller than. Firstly, the transmitter divides data symbols into groups and each group includes -QAM symbols. Then group data can be represented as -point vector after IFFT operation according to Formula (3). Then the transmitter generates the time-domain samples of modulated data (Fig. 1a). For the pilot signal, the transmitter generate $KN$ time-domain samples according to Formula (3) with . The pilot signal also has a property that makes ( ). Therefore, the pilot signal is designed as a known signal to benefit the system performance [2]. That is, the proposed system will utilize the pilot symbol to capture the channel information as well as alleviate the influence of multipath fading. After that, both signals are mixed as shown in the Fig. 1a and transmitted. The effect of multipath channel on the transmitted signal is shown in Fig. 1b. The receiver can utilize the constant signal to achieve promising timing performance [2]. We find that appropriate duration of the received signal for demodulation should be varied from to and then the start point from is an acceptable timing offset. The receiver can utilize the pilot signal to estimate the channel information. Such pilot signal can be removed from the received signal after the equalizer and IFFT operations. Then the receiver rebuilds the time-domain OFDM samples according to Formula (3). After that the samples are operated by FFT again to obtain the estimated frequency-domain samples, the receiver can demodulate the data symbols. IV. PERFORMANCE OF PROPOSED OFDM SYSTEM We choose the simulated BER and PER performances of the proposed system over the fading channel model of JTC 94 (indoor residential B) [3] in Fig. 2. We assume that the MMSE equalizer adopts perfect channel information. The supported symbol rates are 250 k symbols/s for and 62.5 k symbols/s for. The results show it can